Hello everyone,
I would like to have your advice for
a specific issue. I could not decide on whom to discuss with, but I believe
that your experiences may help me on the subject. I am designing a DAQ system with PCI-6123 cards.
I work on power electronics and
power systems. I usually have to deal with racks, cablings, controller card
designs, PCB implementations which are in fact do not seem to be directly
related to my area, but they are. We usually have EMI and coupled noise
problems in our circuits.
My confusion is about shielding and
grounding. By this, I mean,
1) Where the shield conductor of a
shielded twisted pair cable should be connected? Should it be Earthed, or
should it be connected to the reference voltage point of the signal generator?
2) How should a metal enclosure
covering a PCB employed? Should it be connected to Earth or analog ground of
the PCB?
I have read thorough lots of
documents and books, but the subject is still not clear. In “Noise Reduction
Techniques in Electronic Systems” of Henry W. Ott and “Grounding and Shielding
Techniques in Instrumentation” of Ralph Morrison, the causes of noise and
interference are covered in detail and are well understandable. Shielding
against magnetic interference (twisted pair or coax cable, steel enclosure
rather than aluminum or copper, etc.) is also clear. However, shielding against
electric fields (electrostatic effects) is rather confusing. It is mentioned to
connect the shield of a cable to the ground. The word “ground” means so many
things. Is it the Earth, or the reference voltage point of the DAQ
(measurement) system, or the reference voltage point of the signal generator
(or power supply)? This is not clear.
In an application note by Alan Rich of
Analog Devices it says: “An electrostatic shield, to be effective, should be
connected to the reference potential of any circuitry contained within the
shield. If the signal is earthed or grounded (i.e. connected to a metal chassis
or frame, and/or to earth) the shield must be earthed or grounded. But
grounding the shield is useless if the signal is not grounded.” The schematic is given (please see the attachment) and it is said that D is the best solution because no noise
voltage can be coupled to V12.
This paragraph is very clear indeed.
However, we have purchased some equipment whose input and output connections
are made with all shielded cables. The manufacturer recommends that the whole
equipment should be placed in a 19” rack, the rack should be well earthed, the
equipment chassis should be well connected to the rack (thus to earth) and all
the shields of the cables that go outside of the rack should be connected to
earth (rack earth bar) also. This sounds reasonable if 1) All the signals are
generated by the equipment inside the rack, 2) The reference voltage point of all
the signals is the Earth. I found out that, the first one is true, however the
other is not. Because I know that the signals are isolated from each other.
This violates that there exist only one reference voltage point. So, what does
shielding do good for?
So the question arises: Are there
any different situations that I should connect a metal enclosure of electronic
equipment to Earth, and all the shields to Earth, although the signals are not
referenced to Earth? By the way, some of my colleagues strictly argue that all
the shields and enclosures should be earthed no matter the signal inside is
earthed or not, depending on their experiences, but I am not satisfied with
their explanations. Does this method solve problems of RF or other
interferences created by broadcast antennas, 50/60 Hz power lines, lightnings,
contactor switchings etc.? Or does Alan Rich’s paragraph cover and take into
account these interferences and should I be sure?
I will appreciate your help very
much. Looking forward to your reply.
Best Regards,
Cem Özgür
